1. Field of the Invention
The present invention relates to a polymer secondary battery making use of a gelled solid electrolyte or other secondary batteries, and their production process.
2. Prior Art
Various batteries are now used in diverse fields from electronics to automobiles. These batteries, for the most part, contain a liquid electrolyte, and so firm sealing is required for the purpose of preventing electrolyte leakage. In lithium ion secondary batteries having wide applications as driving power sources for portable devices chiefly because of achieving high energy densities, too, firm metal cans are unexceptionally used as battery cans for the purpose of preventing electrolyte leakage. In other words, the lithium ion secondary batteries cannot make full use of their merit contributing to weight reductions. In consideration of the entire spectrum of existing devices, weight reductions and downsizing are an unavoidable problem. In devices utilizing existing lithium ion secondary batteries, however, the whole weight proportion of the batteries in the devices is on the increase. In addition, battery thickness places some limits to the downsizing of the devices. Thus, it is no exaggeration to say that one of keys to the future development of lithium ion secondary batteries is their weight reductions and downsizing.
Situation being like this, lithium polymer secondary batteries are now under development. In a lithium polymer secondary battery there is no free liquid at all with no concern about electrolyte leakage, because a gelled solid electrolyte with a polymer swollen by a liquid electrolyte is used. The lithium polymer secondary battery attracts attention as the coming generation of a battery for some additional reasons that it can be downsized to an ever thinner form, it can be minituarized through lamination, and it can have a high degree of freedom in shape selection. For instance, U.S. Pat. Nos. 5,296,318 and 5,418,091 disclose a gel electrolyte wherein 20 to 70% by weight of a solution with a lithium salt dissolved therein is contained in a copolymer or P(VDF-HFP) of vinylidene fluoride (VDF) and 8 to 25% by weight of propylene hexafluoride (HFP), and a lithium-intercalation battery.
Known for the production of the gelled solid electrolyte are two production processes as explained below.
According to the first production process that is a common process, a liquid electrolyte or the like is mixed with a solution of a polymer dissolved in a solvent. Then, the mixture is coated on a substrate by various coating techniques. Finally, the solvent is volatilized off to obtain a gelled solid electrolyte film. It is also proposed to dissolve a polymer in a liquid electrolyte, and then coating or extruding the solution to obtain a gelled solid electrolyte film. However, liquid electrolytes used for electrochemical devices are generally lacking an affinity for water. When the gelled solid electrolyte is industrially produced using these processes, therefore, all process steps must be maintained in a dry atmosphere of the order of the dew point minus 30° C., resulting in some considerable capital investment and maintenance spending.
The second process is typically set forth in U.S. Pat. No. 5,418,091. According to this process, a plasticizer is added to a polymer solution, which is in turn coated on a substrate. Thereafter, the solvent is volatilized off for film preparation. The plasticizer is then extracted from the film to form a porous film. Finally, pores formed by the extraction of the plasticizer are impregnated with a liquid electrolyte. When a battery is produced by making use of this process, a cathode and an anode are first stacked on upon another with the plasticizer-containing porous film interleaved between them. Then, a collector is stacked on and thermocompressed to the electrode assembly to form a multilayer structure. Finally, the porous film is gelled by the extraction of the plasticizer and impregnation with the electrolyte. With this process, capital investment and maintenance spending can be greatly cut down because all steps prior to the electrolyte impregnation step can be carried out in ordinary environments. In addition, the porous films can be easily placed under inventory control because the porous films after coating and drying or the extraction of the plasticizer can be stocked in a film state. For this process wherein the porous film is impregnated with the electrolyte while it is intercalated in the multilayer structure, however, an electrolyte-permeable expanded metal must be used for the collector that is the outermost layer of the structure. The “expanded metal” used herein is understood to refer to a metal sheet with a number of pores formed therein. The expanded metal, because of being susceptible to deformation by external force when it is thin, must have a certain thickness for use with a battery. A problem with a lithium polymer secondary battery using the expanded metal is therefore that it is heavier than a lithium secondary battery using a liquid electrolyte, saving the weight of a battery can. For a battery arrangement wherein the expanded metal comes into direct contact with electrodes, an electrically conductive coating material with a conductive agent dispersed in a resin must be coated on the expanded metal, as typically set forth in U.S. Pat. No. 5,554,459, because it is impossible to ensure uniform electrical connection between both. Moreover, the use of the expanded metal gives rise to non-uniform pressure profiles upon thermocompression. This, combined with the low strength of the gelled solid electrolyte, causes many internal shorting upon thermocompression, offering an obstacle to mass production.
As explained above, the prior art production processes of lithium polymer secondary batteries have some grave problems. These problems are a leading reason why polymer secondary batteries cannot be put to practical use although many years have passed since their proposal. For industrial utilization of lithium polymer secondary batteries, it is thus an important challenge to establish a reasonable production process, although various material problems are of course important.
In view of such situations as mentioned above, one object of the invention is to provide a process that enables a thin and lightweight polymer secondary battery or other secondary battery to be produced easily yet at low cost. Another object of the invention is to provide a secondary battery produced by this production process.